Methods and apparatuses are known in which a load, such as a work piece, or other item to be lifted and moved, are lifted with suction lifters for subsequent transportation during manufacture. After the work piece or lifted object has been moved to a desired position, and lowered onto an appropriate supporting surface, the work piece or object is released from the vacuum lifter for subsequent processing.
Generally such vacuum lifting devices include a cylinder-piston arrangement in which the act of lifting the load generates a vacuum within a fluid cavity defined by the cylinder-piston arrangement. Vacuum developed in the fluid cavity is communicated to a suction cup-type device which grips the load during the lifting operation.
In order to provide for effective and efficient operation of such a lifting device, it is generally desirable that the lifting device be entirely self-contained, with regard to: generating sufficient vacuum at the start of a lifting stroke to allow the lifting device to grip the load; maintaining sufficient vacuum during the lifting stroke; and automatically releasing the vacuum grip on the load at the end of the lifting stroke, when the load has been safely placed on a support, so that the lifting device may be readily lifted away from the load. It is also desirable that at the completion of the lifting stroke, the lifting device be automatically reset, without any intervention from an operator, to a condition whereat the lifting device will be in proper configuration for a successive lifting stroke after being lowered into contact with another load.
In order to provide for automatic sequencing of application and release of vacuum, some prior vacuum lifting devices have included a mechanically actuated control valve, located integrally within the cylinder-piston arrangement, and actuated by relative motion between components of the cylinder-piston arrangement.
In one prior approach to providing such a vacuum lifting device, as disclosed in U.S. Pat. No. 3,347,327 to Engelen, a vertically oriented cylinder circumscribes a piston head suspended from a rod, with the upper end of the rod extending through the cylinder and terminating in a point of attachment to a crane hook or other hoisting apparatus. The lower end of the rod terminates in an enlarged extremity receivable in a depression in the bottom of the cylinder. The depression in the bottom of the cylinder is connected in fluid communication with a suction cup affixed to the bottom end of the cylinder.
The enlarged extremity, at the lower end of the piston rod of Engelen, also functions to block a series of vents extending through the piston head, when the enlarged extremity is bearing against the piston head. When a lifting force is applied to the upper end of the rod of the lifting device of Engelen, the enlarged extremity at the lower end of the rod is pulled into contact with the lower surface of the piston head in a manner blocking the vents through the piston head, such that as continued lifting force is applied, the enlarged extremity on the lower end of the piston rod exerts force against the piston head and pulls the piston head upward within the cylinder. As the piston head moves upward in the cylinder with the vents through the piston head closed, a vacuum is created in the depression in the lower end of the cylinder, which is communicated to the suction cup for gripping and lifting the load. At the end of the lifting stroke, as the load is lowered onto a support, the rod and piston head move downward in the cylinder. When the piston head engages the bottom end of the cylinder, the rod continues to travel downward in a manner allowing the enlarged extremity at the lower end of the rod to move away from the piston head, to thereby open the vents through the piston head and release any remaining vacuum in the depression at the bottom of the cylinder.
The lifting device of Engelen also includes a series of splines on the exterior surface of the rod. The splines, in combination with a ratchet and pawl mechanism mounted at the upper end of the cylinder and including a collar which is rotated 45 degrees on alternate strokes of the rod with respect to the cylinder by the action of the ratchet and pawl mechanism, preclude generation of vacuum within the cylinder on alternate applications of lifting force to the upper end of the rod, so that the lifting device may be lifted free of the load.
The approach taken by Engelen is undesirable for several reasons. First, the arrangement of Engelen requires that the automatic control valve components of Engelen must be load-bearing during both a lifting and a non-lifting application of lifting force to the lifting device. Most notably, when lifting a load, the entire weight of the load and the lifting device of Engelen must be supported by the enlarged extremity at the lower end of the rod through contact of the sealing surface of the enlarged extremity with the lower surface of the piston head. Such an arrangement is highly undesirable, in that the sealing surface is subjected to loads far in excess of what would be required for merely sealing the vents through the piston, thereby leading to reduced reliability and shortened operating life.
On a non-lifting stroke, of the lifting device of Engelen, the upper ends of the splines on the outer surface of the rod must carry the weight of a substantial portion of the lifting device by contacting the lower surface of the rotatable collar of the ratchet and pawl mechanism. Thus, even in a non-lifting mode of operation, the automatic control valve components of Engelen are subjected to high loads, which may cause premature wear and reduced operational life.
In addition, the various components of the automatic control valve of Engelen cannot be removed, for repair or replacement, without total disassembly of the lifting device of Engelen.
Another prior approach to providing a vacuum lifting device, having an automatic control valve, is shown in U.S. Pat. No. 3,431,010, to Glanemann. In Glanemann, an external collar of a control valve housing, extending from the lower end of a piston rod, is pulled into contact with an internal collar of a movable piston, in such a manner that the combined weight, of the lifting device of Glanemann and the load is entirely supported by the interaction of the internal collar of the piston with the external collar of the valve housing portion of the piston rod. A cycling valve arrangement, extending from the lower end of the piston includes a valve disc which opens and closes a hole in the bottom of the piston, on alternate strokes of the piston, for controlling generation and application of vacuum as lifting force is applied at the upper end of the piston rod.
The piston of Glanemann is vertically movable within a cylinder housing, and operatively attached to a crane or other source of lifting force by the valve housing portion of the self-cycling piston valve. A rolling diaphragm is utilized to seal the juncture between the cylinder housing and the piston, in such a manner that when the piston is raised by the valve housing of the self-cycling piston valve, while the piston valve is in a closed position, a vacuum is generated inside the cylinder housing in a space below the piston. The vacuum generated in the space below the piston is communicated to one or more suction pads via vacuum hoses or conduits.
Although the lifting device of Glanemann improves somewhat upon the lifting device of Engelen, by not requiring the valve disc of the control valve of Glanemann to be load bearing, as was the case for the enlarged extremity at the lower end of the rod of Engelen, the valve housing portion of the lower end of the piston rod of Glanemann must still be fully load bearing. In addition, by locating the working components of the valve apparatus of Glanemann within the lower end of the piston rod, the piston rod becomes structurally less efficient than if the control valve components were located elsewhere. Furthermore, as was the case in the lifting device of Engelen, the working components of the control valve of Glanemann are located internally to the lifting device in such a manner that they cannot be removed for repair or replacement without total disassembly of the lifting device of Glanemann.
Also, as was the case with the lifting apparatus of Engelen, because the entire weight of the load and lifting device of Glanemann must be supported through the self-cycling piston valve assembly, rather than directly through structural elements of the lifting device, such as the cylinder housing, the lifting capacity of the lifting device of Glanemann is significantly less than it would be if the weight of the load were carried through other structural components such as the cylinder housing. Also, should a leak occur, as might be experienced due to pinching or puncturing of the rolling diaphragm or suction cup, or the presence of dirt between the suction cup and load, vacuum in the cylinder housing below the piston, in Glanemann, can be significantly and rapidly reduced in a manner that may result in safety concerns if such a loss of vacuum occurs during a load-lifting cycle.
In another prior approach to providing a vacuum lifting device having an automatic control valve, as disclosed in U.S. Pat. No. 3,785,691, to Sperry, a lifting cylinder assembly includes a main cylinder body adapted at an upper end thereof for attachment to a hook means, and a piston plate disposed within the main cylinder body for generating a vacuum within the cylinder body above the piston plate. A piston rod and plunger assembly extends downward from the piston plate and out of the main cylinder body for attachment at a lower end thereof to a suction cup device.
The piston rod and plunger assembly includes a tubular piston rod portion thereof, fixedly attached at an upper end of the tubular piston rod to the piston plate. The plunger rod is inserted into the lower end of the tubular piston rod, and attached to the tubular piston rod by means of a cylindrical pin extending through the plunger rod, with opposite ends of the pin projecting outwardly into opposed slotted openings in the tubular piston rod, to thereby slidingly interconnect the plunger to the cylindrical piston rod in a manner allowing the plunger to move up and down a limited distance within the tubular piston rod.
A first valve means, of Sperry, is located within the tubular piston rod in the hollow space above the upper end of the plunger rod and below the piston plate. The first valve means includes a valve seal which bears against a lower surface of the piston plate to close off a vent opening through the piston plate when it is desired to produce vacuum within the cylinder above the piston plate. With the arrangement of Sperry, however, a second vent valve is also required to close off a counter-sunk valve seat in the main cylinder body, in cooperation with the first valve means, in order for vacuum to be produced.
The first valve means of Sperry is a complex arrangement of toothed upper and lower cams having slots in an outer periphery thereof for engaging an index pin, and a pair of biasing springs, all located within the upper end of the tubular piston rod, and therefore movable with the piston plate and piston rod. Actuation of the first valve means of Sperry is accomplished through relative movement of the plunger rod with respect to the tubular piston rod, as the lifting device is raised and lowered, by virtue of axial movement of the cylindrical pin connector within the slotted openings of the tubular piston rod.
Although the lifting device of Sperry eliminates the disadvantages of the rolling diaphragm of Glanemann, and does so without having the valve seal of the first valve means be load bearing, as was necessary with the arrangement of Engelen, the approach of Sperry does so through the use of an even more complex valve arrangement moving with the piston plate and piston rod, and buried deeply therewithin, in such a manner repair and replacement of the control valve, once again, cannot be accomplished without total disassembly of the lifting device of Sperry. Furthermore, as was the case with the lifting devices of Engelen and Glanemann, the tubular piston rod and plunger arrangements required by the first valve means of Sperry must be load-bearing. The need for locating the first valve means within the piston rod, thereby requiring the piston rod to be a hollow tube, and the further need for providing the slotted openings and plunger arrangement connected with the connecting pin in Sperry, significantly reduce the load carrying capability of a lifting device of the type taught by Sperry.
Furthermore, in addition to the disadvantages discussed above, the arrangement of Sperry requires the second valve means to generate vacuum for lifting the load. The necessity for the second valve, together with the highly complex nature of the first valve means of Sperry, add considerable additional complexity to the overall construction and operation in a manner increasing the size dead-weight, and cost of a lifting device, according to Sperry, together with an attendant reduction in reliability.
What is needed, therefore, is an improved method and apparatus for effectively, efficiently, and safely constructing and/or utilizing a vacuum lifting device.